|Pea plants infected with Fusarium oxysporum. Note the downward curl of the leaves on the plant that is still green, and the progression of the yellowing of leaves from the plant base toward the tip on the plant with the more advanced disease. Photo by Eric Sideman.|
By Eric Sideman, Ph.D.
Have you ever had a row of peas give up just before producing peas? Nothing is as discouraging as watching a crop grow and look good for weeks and then, shortly before you get to reap any fruit, wham, things turn ugly quickly.
Ah yes, last year nearly all of us saw our tomatoes take that route. Thankfully, late blight is rare; but peas are a different story. Some of us commonly see the demise of our peas.
Fusarium wilt of peas is a fungal disease that first appears as downward curling leaves, which become pale green and flaccid, then turn yellow, starting at the bottom and progressing upward until the entire plant it is yellow – and eventually become dry and crisp.
The root system may be normal. If not, and if the plant in the early stages of the disease is easily pulled from the ground because it has a very small root system, then the disease may be Pythium root rot. However, Pythium and Fusarium can be difficult to distinguish, because once Fusarium sets in, the roots may be invaded by other pathogens, including Pythium, and even a lab will find too many kinds of fungi to tell which one caused the problem.
Ten or more races of Fusarium oxysporum can cause wilting of peas; Races 1 and 2 are the common ones in New England. A race is a subgroup of a species of fungus or other pathogen, similar to a variety of a crop. Race 1 causes true Fusarium wilt. The other races cause what is called “near wilt,” which looks like wilt to me. Knowing the race is important, because the best way to avoid the disease is to grow resistant varieties of peas – but a variety that is resistant to Race 1 may not be resistant to Race 2 at all.
Fusarium is a soil-inhabiting fungus, surviving from year to year in the soil as thick-walled, very hardy spores that can sit in the soil surviving all kinds of conditions for more than 10 years. When a susceptible plant is growing nearby, the fungus penetrates the roots and grows into the vascular system, interfering with water movement, hence causing wilt. The disease progresses rapidly in soil that is 70 degrees or warmer, so gardeners typically do not see a problem until June. As the fungus grows in the plant, it produces large numbers of microscopic spores in and on stems, roots and other plant parts.
Fusarium oxysporum is disseminated through transport of contaminated soil on boots and equipment, infected crop debris on trellises and stakes, and infected seeds. The disease may start in single plants scattered around the field. A year or two later, circles of diseased plants may occur, and these circles may coalesce as they grow larger over the years. When conditions are ideal for the pathogen, i.e., when the soil is warm, entire fields may be destroyed. Plants die more quickly when conditions are dry.
Crop rotation and sanitation will help manage the spread and buildup of pea wilts but are of limited value because of the persistence of the spores once they get into the soil. Use long rotations with non-pea crops. Clean trellises and stakes, physically removing any debris, to avoid bringing infected pea debris to a new area or field. By far the best control of Fusarium wilt of peas is use of resistant varieties.
Eric Sideman is MOFGA’s organic crops specialist. You can address your questions to him at 568-4142 or [email protected].